The present invention relates to apparatus for detecting the presence of an object within a surveillance zone and, more particularly, to apparatus employing either a varying magnetic field for detecting a body of high permeability material or a microwave field for detecting a suitable receptor-reradiator.
Surveillance systems based upon detection of high permeability materials have been known publicly since at least as early as May 4, 1934 when French Pat. No. 763,681, granted to Pierre Arthur Picard, was published. Essentially, said patent describes apparatus wherein a first winding or coil, the transmitting coil, is powered by a source of alternating current, and a second winding or coil, the receiving coil, is disposed in registration with the first coil and coupled to a filtered detecting circuit. One of the coils is in the shape of a figure eight while the other is rectangular and encircles the figure eight coil. Picard refers to this arrangement as an "electrodynamic balance" since in the absence of a disturbance within the generated field there will be zero net energy transferred between the two coils.
Considerable effort has been directed toward improving upon both the system electronics and the field coupling coils as disclosed in said Picard patent. With regard to the electronics, reference may be had to U.S. Pat. No. 4,309,697, issued Jan. 5, 1982, to John N. Weaver, filed Oct. 2, 1980 for "Magnetic Surveillance System With Odd-Even Harmonic and Phase Discrimination," and assigned to the same assignee as the present invention. There is described therein an improved system of the foregoing magnetic field type wherein the apparatus creates a surveillance field varying sinusoidally at a fundamental frequency. Parallel filter channels select the second and third harmonic components of the perturbations created by the high permeability material (sometimes referred to as a marker) when present in the field, and the phase of the second harmonic relative to a reference signal is compared. The signal level of each of the second and third harmonic components as well as the aforesaid phase congruency cooperatively control an alarm circuit.
While various attempts have been made to improve upon the coil elements, they all, with but one exception, rely upon placing the coils in fixed positions generally parallel to, either on the same side or on opposite sides of, a pathway to be guarded. The problem associated with fixed or stationary coils is that they must be able to transmit to and receive from a three dimensional space, the surveillance zone, in a homogeneous manner. Heretofore, this has been accomplished by increasing the transmitting power. Unfortunately, the greater the power the more difficult it becomes to limit the field to the desired zone. A "backfield" area is developed which gives rise to detection of markers outside of the desired zone. Such increased power also demands that large ferrous objects and certain electronic instruments be located a substantial distance from the coils. This constraint limits the usefullness of such systems where items with markers attached must be kept in close proximity to the coils. Moreover, most retail establishments where this type of system could be otherwise used to advantage have checkout counters constructed of large, thin sheets of ferrous material. In such locations no significant "backfield" can be tolerated.
When the "backfield" is reduced by reducing transmitter power, sensitivity of marker detection is sacrificed. Entire regions within the intended surveillance zone will have insufficient field intensity to accomplish detection. At least such was the case prior to the present invention.
Further by way of background said Picard patent states that it is possible to cause the person who is liable to steal an object to pass into the interior of one of two balanced coils, but that a better arrangement would consist in forcing the thief to pass very close to one side of the door comprising the windings. For example, one can mount the frames in the turning portion of a revolving door. Unfortunately, said Picard patent contains no illustration or description as to how such modification might be accomplished and the reader is left to conjecture as to how an operative embodiment might be constructed. It can only be assumed that the intent of the patentee was to utilize a coil geometry of the type described elsewhere in his patent. Thus, it would appear to be in the form of one figure eight coil surrounded by an encircling winding all in the same plane. Designs of this type where transmitting and receiving coils lie within one another in the same plane are characterized by a backfield equal to the front field and by significant edge fields normal to the coil axes that can produce detection of markers.
Numerous deficiencies can be found in the Picard disclosure. First, the usual revolving door normally is constructed with glass panels mounted within metal frames secured to a center post. A metal frame in this environment could provide a short circuit loop surrounding the surveillance coils interfering with their operation. Overranging would extend beyond the confines of the cylindrical portion of the door casement. Hence, the adjacent room area would be swept by the field as the door revolves. Moreover, a revolving door is intended as an interface between the interior and exterior of a room or space and is not particularly suited for apprehension of stolen goods or thieves. It also has physical limitations unsuited to the passage of wheelchairs or shopping carts, for example.
With the foregoing in mind, it is one object of the present invention to provide an improved apparatus of the foregoing general type capable of covering a narrow corridor or passageway with essentially no backfield sensitivity such as might be used in the space between checkout counters in a modern supermarket.
Another object of the present invention is to provide a magnetic surveillance system that can be employed in association with a supermarket checkout counter or the like for controlling the checkout lane while manifesting immunity to interference from adjacent cash registers and other electrical apparatus as well as from harmonic interference generated in the metal of the counters.
Besides using high permeability markers for surveillance, systems are known that make use of the signal mixing and reradiation capability of a semiconductor diode. As an example, reference can be had to Gordon et al. U.S. Pat. No. 3,895,368, issued July 15, 1975, for "Surveillance System and Method Utilizing Both Electrostatic and Electromagnetic Fields," and assigned to the same assignee as the present invention. Said patent describes a system wherein a microwave signal generator projects an electromagnetic wave into a space under surveillance to establish a first field. A pulse or frequency modulated low frequency generator is used to apply a voltage to a discontinuous conductor for establishing a second field, electrostatic in nature, throughout the space. Presence in the space of a miniature passive electromagnetic wave receptor-reradiator in the form of a semi-conductive diode connected to a dipole antenna causes the reradiation of the low frequency component modulated on the microwave component as a carrier. The front end of a receiver system is tuned to the microwave frequency and feeds a suitable detector circuit responsive to the low frequency signal. A coincidence circuit energizes an alarm circuit whenever the detected signal coincides with the original modulation envelope being applied to the low frequency generator. The discontinuous conductor may be a foil sheet or the like.
In said Gordon et al. patent it is contemplated that the discontinuous conductor as well as the usual antennas for transmitting and receiving the microwave signal are all mounted in one or more stationary pedestals or the like. We are not aware of any prior suggestion that the antennas might be moveable. However, in order to limit the surveillance zone to a confined region while maintaining reliable detection there are certain situations for which the known systems are inadequate.
It is, therefore, a further object of the present invention to provide an improved microwave surveillance system capable of covering a confined region with increased reliability.